1) Field of the Invention
This invention relates in general to dual-tone photoresists. In one aspect, this invention is directed to photoresist formulations which produce a positive or negative image depending upon processing conditions. In another aspect, this invention relates to liquid, positive photoresist formulations having improved high temperature stability. The invention also provides formulations containing certain di-substituted carbodiimides as resolution enhancement reagents.
2) Description of the Related Art
Photoresists are photosensitive materials which change their solubility after exposure to light. They are typically novolac resins which have a diazonaphthoquinone (DANQ) sensitizer attached to the polymer backbone or added to the resist formulation. Diazonaphthoquinone sensitizers also act as inhibitors which decrease the solubility of diazonaphthoquinone-novolac films in basic solutions.
A dual-tone photoresist is defined as a photoresist capable of producing positive or negative images depending upon processing conditions. In 1978, H. Moritz and G. Paul described in U.S. Pat. No. 4,104,070, that the addition of monazoline (1-hydroxyethyl-2-alkyl imidazoline) to a positive working photoresist formulation allowed image reversal (negative images) to be obtained. S. A. MacDonald et al, Interface, p. 114 (1982), reported further modification of this process by using a similar additive, imidazole. Other basic chemicals such as triethanolamine and gaseous amines have also been reported as having the ability to cause image reversal.
If liquid bases are added to photoresist formulations, they have poor formulation stability. As a result, potential commercial dual-tone photoresists users treat photoresist films with gaseous amines, after exposure, to cause reversal. The use of gaseous amines, however, requires baking the resist in the presence of a poisonous gas. To date, dual-tone photoresists are still being commercialized by some fabricators who use this technique.
Recent studies have shown that image reversal could be accomplished by adding polycarbodiimides to a diazonaphthoquinone-novolac photoresist formulation. Images with good heat stability were demonstrated. However, the formulations were not stable.
Accordingly, one or more of the following objects will be achieved by the practice of this invention. It is an object of this invention to provide a novel photoresist formulation. A further object of this invention is to provide a photoresist formulation of the novolac-type which has improved stability. A still further object is to provide a formulation which avoids the use of dangerous gases in its preparation. A further object of this invention is to provide a photoresist formulation containing a di-substituted carbodiimide. Another object of this invention is to provide a method for the preparation of the formulations and their use in the preparation of positive or negative images. Another object of this invention is to provide dual-tone photoresist formulations from which positive and negative images can be prepared with linewidths of from about 3 microns or less, to about 5 mils or higher. A further object is to provide photoresist formulations which can be used for imaging films of thicknesses of from about 1.5 to about 5.5 microns. A still further object is to provide photoresist formulations for use in preparing images wherein exposure energies range from about 89 millijoules per square centimeter for semiconductor applications to about 140 millijoules per square centimeter for circuit board applications. These and other objects will readily become apparent to those skilled in the art in the light of the teachings herein set forth.